1. Field of the Invention
The present invention relates to an approach detecting system which detects the position of a target using a cable-shaped electric wave transmit/receive means.
2. Background of the Invention
Conventionally, approach detecting systems for detecting an approach of a target using a leaky transmission line which is called a leaky cable are known. As such a conventional approach detecting system, there has been provided, for example, a system in which leaky transmission lines are placed on the periphery of a site to be monitored for receiving electric waves using these leaky transmission lines, and for detecting a target by detecting variations in the electric field which occur when the target is approaching to the leaky transmission lines (refer to, for example, patent reference 1).
In addition, a conventional approach detecting system which has such a structure as mentioned above and which can also detect the position of a target with respect to a direction distant from leaky transmission lines is also disclosed (refer to, for example, patent reference 2). This conventional approach detecting system is implemented with a reflector for reflecting electric waves toward the far ends of the leaky transmission lines.
Furthermore, there has been provided a conventional approach detecting system which is so constructed as to observe targets by alternately emitting out electric waves of different frequencies using, as a transmit cable, a leaky coaxial cable having a radiation directivity which varies with frequency, and determine the position of a target which is staying in a plane which is sandwiched by the leaky cable for transmission and a leaky cable for reception using an observed distance difference (refer to, for example, patent reference 3).    [Patent reference 1] JP,10-95338, A    [Patent reference 2] WO No. 05/013223 pamphlet    [Patent reference 3] JP,2004-125604, A
However, there are the following problems with the above-mentioned prior art systems.
For example, the conventional approach detecting system as disclosed in patent reference 1 can measure only a distance of each target with respect to the direction of the leaky transmission lines, but cannot calculate a distance of each target with respect to a direction which is apart from the leaky transmission lines. A problem with the conventional approach detecting system is therefore that when an object which should not be originally detected is staying at a far distance from the conventional approach detecting system, it can be detected accidentally if its size is large and the level of an electric wave reflected from the object is large.
A problem with the conventional approach detecting system as disclosed in patent reference 2 is that it can measure the position of only one target with respect to a direction apart from the leaky transmission lines, but cannot measure the positions of two or more targets individually. For example, when two targets are staying in a specific area, electric waves reflected from the two targets are combined into a single electric wave and this electric wave is then reflected by the open end of the leaky transmission line for reception. Therefore, it is difficult to distinguish the distance (with respect to a direction perpendicular to the leaky transmission lines) between one of the two targets and the leaky transmission lines from the distance between the other one and the leaky transmission lines. Another problem with the conventional system as disclosed in patent reference 2 is that the distance (with respect to a direction perpendicular to the leaky transmission lines) from the leaky transmission lines cannot be determined unless the target is staying at a certain distance or longer from the leaky transmission lines. This is because although the conventional system calculates the distance (with respect to a direction perpendicular to the leaky transmission lines) from the leaky transmission lines by observing a target component which appears far away from the signal component of the combined wave at the end, the target component is covered by the combined wave at the end and therefore cannot be detected unless the target is considerably apart from the leaky transmission lines because the combined wave at the end has a very large intensity.
Furthermore, a problem with the conventional approach detecting system as disclosed in patent reference 3 is that when two or more targets are approaching to a specific area, it cannot measure the positions of the two or more targets correctly. Another problem with such the conventional approach detecting system is that only a target which is staying between the two leaky coaxial cables can be detected theoretically, while there is no guarantee that it operates normally for targets which are staying outside the two leaky coaxial cables and it cannot detect the position of any target staying outside the two leaky coaxial cables. A further problem with the conventional approach detecting system as disclosed in patent reference 3 is that no electric wave is outputted at all from the leaky cable for transmission or electric waves are simultaneously outputted from the leaky cable for transmission in two or more directions in a case in which the frequency of a signal applied to the leaky cable for transmission is badly chosen, and therefore the position of any target cannot be measured in such a case in which the frequency is badly chosen.